1. Field of the Invention
The present invention relates to a one-piece solid golf ball or a two or more piece solid golf ball which comprises an inner core and a cover covering the inner core.
2. Description of the Related Art
There are one-piece solid golf balls and two or more piece solid golf balls golf balls commercially available. The one-piece solid golf balls are formed by molding and vulcanizing a rubber composition. The two or more piece solid golf balls are composed of an inner core and a cover covering the inner core. The inner core is composed of either one uniform rubber material or a combination of a center and one or more outer layers disposed over the center. At least one portion of the inner core is formed by molding and vulcanizing the rubber composition. The rubber composition for the solid golf balls generally comprises polybutadiene rubber having at least 40% of cis-1,4 structure as a base rubber, metal salt of .alpha.,.beta.-ethylenically unsaturated carboxylic acid (e.g. methacrylic acid, acrylic acid, etc.) as a co-crosslinking agent and a vulcanizing agent (e.g. organic peroxides).
The one-piece solid golf balls are mainly used at driving ranges. The most important performance for the golf ball used at driving ranges is high durability (i.e. the golf balls must be hard to break).
However, the conventionally used one-piece solid golf balls do not have sufficient durability, and it is therefore strongly desired to enhance the durability.
The two or more piece solid golf balls are generally used for high quality golf balls. It is further required to improve the hit feeling of the balls. It is considered that the poor hit feeling occurs due to the difference in hardness between the cover and the inner core. In order to improve the hit feeling of these golf balls, it has been proposed to make an inner core having a surface portion that is harder than its center portion. The larger the hardness difference between the surface and center of the inner core, the better the hit feeling. However, as this difference in hardness becomes larger, the durability of the ball is liable to become inferior. Sometimes, the golf ball will break after being hit with a golf club (e.g. No.1 to No. 3 wood clubs, etc.) 10 to 15 times.